The present invention relates to an apparatus and method for hardening steel and more particularly to a method and apparatus for quenching steel pipes of substantial and varying thicknesses.
Quenching is one of the oldest and most common methods of hardening steel by heat treatment. It consists of heating the steel above its critical transformation temperature at which a component known as austenite begins to form, and then cooling it fast enough, usually by quenching into a liquid such as water or oil, to avoid any transformation of the austenite until it reaches the relatively low temperature range within which it transforms to a hard martensite. The steel is subsequently reheated or tempered to remove the internal stresses caused by the inherent expansion of the martensite.
The quenching of steel from its critical transformation temperature to the martensitic transformation temperature requires a rather severe cooling rate if the formation of pearlite is to be avoided. Given the importance of the cooling rate in producing the desired properties, the production of large pieces of steel has always presented particular difficulties, since the temperature drop at the center of a given piece of steel lags the temperature drop at the surface.
A number of processes have been developed in an attempt to address this problem. For example, metal alloys, such as manganese, silicon, nickel or chromium have been added to retard the formation of pearlite to allow for an initial lower quench and to enhance in other ways the final properties of the steel. However, the use of alloys adds considerably to the expense of the steel.
A variety of methods and devices have been developed or suggested as ways of more readily controlling heat transfer from both the exterior and interior surfaces of pipes by using water, as well as other substances, as a cooling medium. These prior processes employ a variety of sprays and flow schemes. For example, in U.S. Pat. No. 3,212,766 there is disclosed an apparatus for quenching a long tube. The apparatus comprises a cooling bath, in which a tube is immersed, and a coolant vortical-flow-inducing nozzle. The nozzle forces a vortical flow of coolant through the interior of the tube.
U.S. Pat. No. 3,623,716 discloses an apparatus for hardening long pipes. The pipe is immersed in a tank equipped with a nozzle arranged for introducing a cooling medium into the interior of the pipe in such a manner that cooling medium is drawn from the exterior of the pipe to the interior of the pipe.
U.S. Pat. No. 3,877,685 discloses an apparatus for quenching a steel pipe with a cooling medium including an isolator which is in fluid communication with a retractable nozzle. The isolator and retractable nozzle cooperate so that the relative proportion of cooling liquid passing into the pipe and around the pipe may be varied. The flow of cooling medium is directed along the longitudinal axis of the steel pipe.
U.S. Pat. No. 4,165,246 discloses a process for heat treating steel pipes with a wall thickness ranging from 16 to 36 mm. After the steel pipe is heated, it is passed on rollers to a cooling zone while water directed from nozzles encircling the pipe quench the surface below the martensitic transformation temperature.
U.S. Pat. No. 4,116,716 discloses an immersion cooling apparatus including a cooling tank containing cooling liquid, a mechanism for locking the immersed pipe in position, and a nozzle extending toward the interior of the pipe in the direction of the pipe axis.
These and other devices and methods, which employ a variety of quenching mechanisms using cooling baths and the like, suffer from one or more of several limitations. For example, these devices and methods often fail to provide a sufficiently severe quench, so that the thickness of steel pipe which may be successfully treated is limited. Likewise, the strength and other properties attainable for a given thickness of pipe are limited. Also, many devices and methods do not provide uniform cooling or cannot accommodate a steel piece of varying thickness such as upset pipe. Additionally, sonic devices cannot vary the character of the quench from segment to segment or along the length of a pipe.
These and other limitations of prior processes and methods are substantially minimized if not eliminated by the present invention.